Because there is no such crime in the UK. It's not illegal to be naked in public, any crime all depends on intent.
"In England and Wales, the offence of "indecent exposure" and other sexual offences were replaced by the more specific and explicit Sexual Offences Act 2003, section 66. Current laws apply only to genital exposure with intent to shock those who do not want to see them."
I guess you could argue they are intending to shock, but most people just see it as fun.